Conventionally, a technology is considered in which a travel trace of a vehicle is stored and a road shape is learned based on the travel trace. As the travel trace, a dead reckoning trace and an absolute trace are known. The dead reckoning trace is a trace that a travel vector, which is determined by a detection value of a speed sensor, a gyro sensor, or the like, of the vehicle is chronologically arranged. The absolute trace is a trace obtained by synthesizing the dead reckoning trace with a positioning trace, which is generated based on a received electric wave from a positioning satellite such as a GPS satellite, or the like.
Here, in some cases, by the dead reckoning trace, an accurate data about a distance, a curvature of a curve, or the like may not be obtained to an extent where a travel control of the vehicle is performed well. In addition, the dead reckoning trace does not include absolute coordinates. Therefore, the absolute trace rather than the dead reckoning trace tends to be used for the travel control of the vehicle.
However, since the absolute trace uses the positioning trace based on the electric wave received from the positioning satellite, the following difficulty may occur.
Thus, it may occur that, in a place surrounded by high-rises or the like, for example, the electric wave from the positioning satellite is reflected by buildings or the like, and that a car navigation device receives the electric wave from multiple pathways. This phenomenon is known as a multipath phenomenon. In a place where the multipath phenomenon occurs, the car navigation device can not accurately detect a location of the vehicle, and therefore, the car navigation device can not generate the accurate positioning trace. Thus, when the absolute trace is used as a trace for the travel control of the vehicle, it may be a subject to remove an influence by the multipath phenomenon.
As a technology for removing the influence by the multipath phenomenon, in Patent Document 1, a technology is disclosed in which a receiver sensitivity of a GPS signal is reduced in a place where the multipath phenomenon is prone to occur, so that a positioning calculation is not performed. However, in the described technology in Patent Document 1, since the calculation of the trace is performed in real time when the vehicle travels in the place where the multipath phenomenon is prone to occur, a removal of the influence by the multipath phenomenon may be insufficient.